Various gas phase reaction processes have been developed and put to practical use for surface flattening and the like of electronic devices. For example, a substrate surface flattening method described in patent literature 1 flattens substrate surfaces by irradiating them with monatomic ions of Ar (argon) gas or the like or molecular ions at a low irradiation angle to cause sputtering.
Recently, a method for flattening solid surfaces using a gas cluster ion beam has been drawing attention because of reduced surface damage and capability to reduce surface roughness greatly. For example, patent literature 2 discloses a method for reducing surface roughness by irradiating solid surfaces with a gas cluster ion beam.
In this method, gas cluster ions directed at a workpiece are broken by collisions with the workpiece, causing multi-body collisions between constituent atoms or molecules of the clusters and constituent atoms or molecules of the workpiece, consequently causing conspicuous motion parallel to a workpiece surface, and thus producing a cut in a direction parallel (hereinafter referred to as a lateral direction) to the workpiece surface. This phenomenon is known as lateral sputtering. The lateral motion of particles with respect to the workpiece surface enables ultra-precision flat grinding corresponding to the size of atoms.
A gas cluster ion beam, whose ion energy per atom is lower than in ion etching, enables required ultra-precision grinding without damaging the workpiece surface. This indicates the advantage that solid surface flattening by means of a gas cluster ion beam causes less damage to workpiece surface than ion etching described in patent literature 1.
In the flattening by means of a gas cluster ion beam, it is generally recognized that preferably the cluster ion beam is directed approximately perpendicular to the workpiece surface. This is to make full use of the effect of “surface flattening by means of lateral sputtering.” However, although patent literature 2 described above states that the gas cluster ion beam may be directed obliquely depending on surface conditions such as curves, it does not mention any effect of directing the gas cluster ion beam obliquely. Thus, according to patent literature 2, it follows that the most efficient way to flatten a solid surface is to direct the beam approximately perpendicularly to the surface.
An example of solid surface flattening by means of a gas cluster ion beam is also disclosed in patent literature 3. Patent literature 3 does not state the effect of an angle between the gas cluster ion beam and solid surface on surface flattening, either. In view of the use of “lateral sputtering” effect, it appears that patent literature 3 shows data on vertical irradiation as in the case of patent literature 2 described earlier.
Solid surface flattening by means of gas cluster ion beam irradiation is also reported by non-patent literature 1. Toyoda, et al. irradiated surfaces of materials such as Ge, SiC, and GaN with Ar cluster ions and showed that surface roughness is reduced by the irradiation. Again, the gas cluster ion beam was directed approximately perpendicularly to the surfaces.
On the other hand, non-patent literature 2 describes changes in the roughness of a solid surface when the solid surface is irradiated with a gas cluster ion beam at various irradiation angles. Between vertical incidence on the surface at 90° and irradiation parallel to the surface at 0°, it shows that an etching rate, i.e., speed at which the surface is etched, is the largest at the time of vertical incidence and decreases with decreases in the irradiation angle. Regarding relationship between the surface roughness and irradiation angle, by conducting experiments using irradiation angles 90°, 75°, 60°, 45°, and 30°, it shows that the surface roughness increases with decreases in the irradiation angle. Irradiation angles below 30° were not checked experimentally maybe because it was considered meaningless.
Integrated circuits and other electronic devices as well as optical devices used for optical communications often contain concavo-convex patterns produced by microfabrication on solid surfaces or surfaces of thin-film materials. However, there has been no report on the use of a gas cluster ion beam for flattening of lateral wall surfaces in concave or convex portions of the concavo-convex patterns. This is because it is considered to be difficult to direct a gas cluster ion beam approximately perpendicularly to lateral wall surfaces in concave or convex portions and impossible to flatten the lateral wall surfaces by the mechanism of lateral sputtering.
Recently, it has been found that irradiation angles smaller than 30° reduce surface roughness greatly (non-patent literature 3). This utilizes effect of oblique irradiation different from the conventional flattening mechanism by means of lateral sputtering.    Patent literature 1: Japanese Patent Application Laid Open No. H07-058089    Patent literature 2: Japanese Patent Application Laid Open No. H08-120470    Patent literature 3: Japanese Patent Application Laid Open No. H08-293483    Non-patent literature 1: Jpn. J. Appl. Phys. Vol. 41 (2002), pp. 4287-4290    Non-patent literature 2: Materials Science and Engineering R34 (2001), pp. 231-295    Non-patent literature 3: Jpn. J. Appl. Phys. Vol. 43, 10A (2004), pp. L1253-L1255